During the past decade, matrix-assisted laser desorption/ionization (MALDI) has proven to be a valuable tool in the analysis of a variety of molecules, e.g., biomolecules such as proteins and other organic molecules, and has application in a wide variety of fields such as genomics and proteomics. In many cases, MALDI ion sources are integrated with an analytical device, e.g., a mass spectrometer, for studying the MALDI ionized analyte. Mass spectrometers are instruments that measure and analyze ions by their mass and charge. For the most part, time-of-flight mass spectrometers (“TOF-MS”) are used for this purpose, but other mass spectrometers may be used as well, such as an ion cyclotron resonance spectrometer (e.g., a Fourier transform ion cyclotron mass resonance spectrometer), ion trap mass spectrometers (e.g., a high-frequency quadrupole ion trap mass spectrometer), and hybrid instruments (e.g., a quadrupole/time-of-flight mass spectrometer, QqTOF).
Generally, MALDI ion sources vaporize and ionize non-volatile biological analytes from a solid phase directly into a gaseous phase. To accomplish this, analytes are suspended or dissolved in a matrix of generally a small organic compound which co-crystallizes with the analyte. A sample containing the analyte/matrix mixture is applied to a suitable support, e.g., a sample plate, which is then loaded into an ion source for performing MALDI. It is thought that the presence of the matrix enables the analyte to be ionized without being degraded, solving a problem of other methods. Accordingly, MALDI enables the detection of intact molecules as large as 1,000 kDaltons, and is particularly suitable for the analysis of biological samples such as proteins, peptides, and nucleic acids, which may range in size from 1 kDa to about 1000 kDa.
A laser beam serves as the desorption and ionization source in MALDI. Once a sample is loaded into the MALDI ion source, a laser is used to vaporize the analyte. In the vaporization process, the matrix in the sample absorbs some of the laser light energy causing part of the illuminated matrix to vaporize. The resultant vapor cloud of matrix carries some of the analyte with it so that the analyte may be analyzed. The matrix molecules absorb most of the incident laser energy, thus minimizing analyte damage and ion fragmentation. Samples may be ionized by a MALDI ion source at atmospheric pressure (AP) or in a vacuum.
Once the molecules of the analyte are vaporized and ionized, they are usually analyzed. As mentioned above, this may be accomplished by the use of a mass spectrometer. Accordingly, the vaporized ions are transferred electrostatically and/or pneumatically into a mass analyzer, for example a TOF-MS flight tube, where they are separated. Following separation of the ions, they are then directed to a detector so that the ions are individually detected. Depending on the nature of the analyzer and how it separates the ions, mass spectrometers fall into different categories. In the case of a TOF-MS for example, separation and detection is based on the mass-to-charge (m/z) ratios of the ions. In TOF-MS, detection of the ions at the end of the time-of-flight tube is based on their flight times, which are proportional to the square root of their m/z.
As such, in general, MALDI involves the generation of ions from analytes in a sample, first by embedding the analytes into a matrix to form crystals and then irradiating the analytes with a laser beam, usually a UV light beam, generated by a suitable laser.
In response to the ever increasing interest in the application of MALDI to a wide range of analytical problems, MALDI sample plate formats, including the size and geometry of the plates themselves and the sizes, geometries and positioning of spots within the plates, are ever-changing. For example, in order to increase detection limits, the concentration of a given analyte in a sample may be increased by decreasing the volume of a sample. Spotting samples of smaller volumes onto a MALDI sample plate leads to a sample plate with smaller spots. Also, as more and more samples are analyzed, samples are spotted onto sample plates at higher densities. In fact, in many MALDI methods, a sample plate must be in a high vacuum before ionization is performed. Since a high vacuum takes a significant amount of time to establish in a MALDI ion source, the throughput of such a MALDI ion source is typically proportional to the density of samples spotted on a sample plate. Also, in addition to the ever-changing densities and sizes of spots on a sample plate, sample plates are variable in their size and geometries, and individual samples may vary in their size, shape and position on a single sample plate.
Current MALDI ion sources typically accommodate little variability of sample plate format, and are usually pre-set to ionize samples from a single MALDI plate type, e.g. a single sample plate, a 24-sample plate, or a 96-sample plate.
Accordingly, a need exists for MALDI sources and methods that accommodate sample plates of different sizes and geometries, and samples of variable sizes, shapes and positions on a sample plate. Of particular interest are methods and apparatus that allow a user to configure a MALDI ion source to ionize a sample at a particular position on a sample plate. The present invention meets this, and other, needs.